A conventional boiling water reactor includes a plurality of control rod drives (CRDs) which are effective for selectively inserting and withdrawing control rods into and from the core of the reactor. In one embodiment, the CRDs include a piston against which is provided a pressurized hydraulic fluid, such as water, for providing the force for inserting the control rods during a scram operation. The pressurized fluid is provided to the CRDs by conventional hydraulic control units (HCUs) for selectively controlling the operation of the CRDs.
In order to ensure effective reactor shutdown during scram, the HCUs include conventional scram accumulators which are connected to a common charging water header and are charged with the pressurized fluid for storing the pressurized fluid for use in the scram operation. The charging water header is connected to a relatively high power, for example about 500 shaft horsepower, CRD pump which is run continuously at a high speed sufficient to generate a discharge pressure great enough to maintain the scram accumulators continuously charged to a pressure sufficient to scram the control rods at their required speeds.
During normal plant operation, when the scram accumulators are full, there is no flow through the charging water header. However, the CRD pump discharge pressure must remain high enough to maintain the charging header at a pressure sufficient to keep the scram accumulators fully pressurized. This discharge pressure is significantly higher than that required for the CRD pump to perform its other normal operating function of delivering purge water on a continuous basis to a conventional fine motion control rod drive (FMCRD) found in the CRD. The purge water is provided from the CRD pump to the respective CRDs through a purge water header that is arranged in parallel with the charging water header.
Thus, a significant economic penalty is incurred in this design by having to provide electric power to drive the high power CRD pump continuously at a speed and discharge pressure greater than those required for delivering the normal purge water flow to the CRDs.